I believe you're mistaken here. I think she was always talking to Bernard and that she never actually talked to Arnold that we got to see. Arnold has been dead for 30 years so had she ever talked to him, she'd be a young lady at the time and she would likely recognize that Bernard looks just like him.
It was speculated before the reveal, that the conversations between "Bernard" and Dolores were actually Arnold. We now have that proof, pretty much.
Dolores' conversation with the ghost of Arnold was in the exact same setting as those previously shown interactions...there's also a curiosity to the conversations that isn't present with Bernard. Add to that, that any host interactions Bernard has, take place in the normal labs we see other characters in, so we can now draw the conclusion that Arnold's curiosity with Dolores ultimately got the better of him.
She was, and is, special. There's something cruel and tragic in how Dolores and Teddy have been constrained by Ford in their respective loops.